A. Field of the Invention
The present invention relates to a non-contact optical imaging system for biometric identification.
B. Description of Related Art
There is a great need in today's age of electronic information for increased security and positive identification. For example, this includes reliable identification of purchasers at vendor sites and over the internet, identification of participants in other financial transactions, and identification in access control and anywhere sensitive information must be handled or transmitted. The growing field of biometrics provides one technological answer to this need for reliable identification. Biometric technologies measure and characterize some unique feature of a person's anatomy, which is then used to identify that person.
A number of biometric identification devices already exist. These include fingerprint readers, hand geometry readers, iris imagers, retinal cameras, voice recognition, and face recognition devices. Devices that are more esoteric include fingernail ridge identification and cameras that image the vascular patterns in the earlobe. Probably the most promising, best developed, and most common of these biometric devices are fingerprint readers.
Each of these biometric identification devices has its own problems and strong points. Important features of any biometric device are user comfort, with resulting willingness to use; low false rejection rate; low false acceptance rate; cost; size; speed of operation; and robustness in a wide range of operating environments. Fingerprint readers represent a good mix of all of these features, especially user acceptance.
There are two basic types of fingerprint readers available today, ones with (1) direct-reading sensors and (2) optical sensors. All direct-reading sensors, and all but a few rare specialized optical readers, require users to press the pad of their finger firmly onto a “platen” to be read. In the case of direct readers, the platen is the surface of the sensor itself. The device essentially makes a contact print of the finger surface, relying on the principle that the higher ridges of the finger will touch, or be closer to the surface, of the sensor than the valleys of the fingerprint. Several variations on this technology are currently available.
On the other hand, the vast majority of existing optical readers have a prism or other glass (or possibly plastic) surface on which users place their fingers, and through which the image of the fingerprint is taken. These devices rely on changes in the reflectivity at the platen surface caused by contact with regions of the finger surface to produce high contrast images of the ridges in the fingerprint. The reflectivity changes are due to the difference and discontinuity in the refractive index at the surface between the platen and air.
When a finger (or any object with a refractive index slightly higher that of air) is placed in intimate contact (including, possibly, by wetting) with the platen surface the reflectivity at that location will be reduced. In principle only the high points (ridges) of the fingerprint will make intimate contact with the platen surface and will therefore appear darker than the valleys. The illumination of the platen surface is designed to maximize this effect.
Contact optical biometric identification devices have inherent problems, including large variations in the print pattern depending on finger moistness or oiliness, variations with finger pressure, poor prints or missing sections of the print with dry or overly wet fingers, loss of contrast due to residue, or latent prints left on the platen from prior use. In addition, there is also the security risk of identity theft due to the possibility of a user lifting any latent prints. Many of the problems with optical readers are also present with direct reading sensors because of the need to place the finger onto the platen. In addition, other problems with the direct-reading sensors include fragility (breaking when an object is dropped on the necessarily thin platen), damage due to electrostatic discharge, and high cost.